The specification states that for an <instance_physics_model>, only the instantiated rigid bodies and constraints will be considered for simulation. However, the example in the <physics_model> section shows a combination of two models without any rigid body instances. Does this mean that in this case, all rigid bodies are instantiated? If this is the case, then how can they be connected to a corresponding target node, as this is only specified in <instance_rigid_body>?

The position and orientation of rigid bodies are specified in world coordinates. If the complete transformation of the target <node> is overwritten with these values, should individual transformation elements be discarded and replaced by a single matrix transformation? And is the case where a target <node> is not in world coordinates, but a child element of a <node> with an additional transformation permitted or is it the responsibility of the user to prevent such a scenario?

The physic model example is only in a library. When it is instantiated (not shown) then the rigid bodies and constraints that are declared in the model are also instantiated and can be targeted.

Nodes that are targeted by physics models should be at the root of the visual scene, otherwise the node’s local coordinate system may not be the world coordinate system. Yes its up to the user to create content that makes sense for the use-case.